Add deprecated_func decorator

[glitchheadgit]

Can you give a dict of deprecated functions and there new analogues?
Then we can add f"Use {new_func[deprecated_func]} instead." for raise FutureWarning.

• Closes Tell people they’re using deprecated functions #2505
• Tests included or not required because:
  Didn't add decorator to any function(don't know which are deprecated)

• Release notes not necessary because:
  Too small issue

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ec7f925) 71.99% compared to head (4dc9c86) 71.98%.
Report is 50 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2714      +/-   ##
==========================================
- Coverage   71.99%   71.98%   -0.01%     
==========================================
  Files         107      107              
  Lines       11914    11916       +2     
==========================================
+ Hits         8577     8578       +1     
- Misses       3337     3338       +1     

Files	Coverage Δ
scanpy/_utils/__init__.py	65.64% <50.00%> (-0.10%)	⬇️

[glitchheadgit]

I think this pull request should be labeled as /label no milestone. Can someone take a look?

[flying-sheep]

I think we should rather go with an existing decorator like https://github.com/tantale/deprecated

A PR should modify all deprecated functions and docstrings to rely on the decorator instead.